It is now possible and desirable to transport very high bit rate digital signals over long distances in communications networks. For example, transmission of digital signals at rates of up to 2.4 Gb/sec. or more over distances of up to 100 km. or more is now possible and is actually used in some advanced communications networks, such as those using optical fiber as the transmission medium.
One particularly attractive high speed communications network, which can conveniently handle such large amounts of digital data, uses the so-called asynchronous transfer mode protocol in which packets or cells of digital data are transported and switched through an optical fiber based network. This kind of network is particularly attractive because the cell based nature of switching reduces the cost of constructing and maintaining the network and increases the efficiency with which available bandwidth may be utilized.
As the amount of data carried on each line of a communications systems becomes greater, it becomes increasingly important to take countermeasures against failure and disruptions of service, such as those disruptions which are caused by a break in the optical fibers. The main countermeasures taken against disruption in prior communications systems include provision of a redundant standby line in prior communications systems include provision of a redundant standby line in addition to a normally active line. The standby line carries the same network user data as the active line. Protection switching from the active line to the standby line is invoked in the event of a disruption of service on the original active line. The data sent on the active line and the data sent on the standby line travel physically diverse paths from their source to their destination. This physical diversity results in differing amounts of travel time for the user data from the source to the destination. The arrival of user data at the destination from the active line thus will either lead or lag the arrival of the same data transmitted on the standby line because of different amounts of delay present in the active and standby lines. This difference in delay is caused by things such as different path lengths between source and destination for the active and standby lines and different circuit elements and characteristics in those two lines.
Lagging or leading data streams flowing in the active and standby lines require that special measures be taken to ensure that there is no data loss or error when a receiver switches from an active line to a standby line. Various forms of hitless or errorless switching systems have been proposed but none of them are very applicable to high speed, high capacity communications systems such as ATM communications systems. For example, a technique for accomplishing errorless switching of synchronous digital hierarchy (SDH) signals from one route to another is described in Chaudhuri et al. U.S. Pat. No. 5,051,979. However, errorless switching techniques are not available for ATM signals. Those errorless switching techniques developed for signals such as SDH signals are not readily applicable to errorless switching of ATM signals, particularly because of the asynchronous nature of the ATM signals. Accordingly, an unsatisfied need exists for a suitable errorless switching arrangement for ATM signals.
In addition to a general lack of suitable errorless protective switching arrangement for ATM signals, prior errorless switching arrangements have been costly in terms of a large amount of data storage needed to successfully accomplish errorless switching between active and standby lines. This large amount of data storage takes up a large amount of physical space and produces a great deal of heat which must be successfully dissipated. Accordingly, there is an additional need for an errorless switching system having less data storage to reduce the cost of implementing the switching system, the space it occupies, and the heat it produces.